[lxc-users] How properly to find what consumes memory inside the container.

[Ivan Kurnosov]

Hi,

there is a server that currently runs ~100 containers.

One of those containers is a subject of my interest.

Brief details about the container: it runs ubuntu xenial, and it's a tiny
file server (samba based) with near to no traffic at all.

I have found that after you upload files to that server, the available
memory size is decreased (while the "buff/cache" size stays at 0). And if
you remove the just uploaded files - the memory consumption drops to the
same value as it was before uploading.

Here is a output of the top (sorted by resident memory size, processes with
more than 500kib rss):

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
   48 root      20   0   52048  18880  14428 S   0.0  0.9   0:10.11
/lib/systemd/systemd-journald
18609 www-data  20   0  349208  15404   7516 S   0.0  0.7   0:13.72
/usr/sbin/smbd -D
 7176 www-data  20   0  345500  10720   6720 S   6.7  0.5   0:06.91
/usr/sbin/smbd -D
25124 root      20   0  340104   9624   6744 S   0.0  0.5   0:00.12
/usr/sbin/smbd -D
37541 root      20   0  344828   8012   4520 S   0.0  0.4   0:02.36
/usr/sbin/smbd -D
15593 root      20   0  344352   6368   3444 S   0.0  0.3   0:00.39
/usr/sbin/smbd -D
 2450 root      20   0  336636   4072   1520 S   0.0  0.2   0:06.09
/usr/sbin/smbd -D
25401 root      20   0   40560   3728   3112 R   0.3  0.2   0:00.49 top
 2447 root      20   0  336636   3528    976 S   0.0  0.2   0:04.30
/usr/sbin/smbd -D
25287 root      20   0   19972   3044   2872 S   0.0  0.1   0:00.01 bash
 2476 root      20   0  238728   2944   1336 S   0.0  0.1   0:28.52
/usr/sbin/nmbd -D
25271 ivan      20   0   21328   2784   2764 S   0.0  0.1   0:00.04 -bash
24250 root      20   0  858936   2616      0 S   0.0  0.1   0:01.98
/usr/sbin/collectd
 2448 root      20   0  426848   2504     20 S   0.3  0.1   0:01.65
/usr/sbin/smbd -D
    1 root      20   0   37884   2488   1676 S   0.0  0.1   0:17.13
/sbin/init
25285 root      20   0   51660   2404   2400 S   0.0  0.1   0:00.00 sudo su
25270 ivan      20   0   95368   2172   1960 S   0.0  0.1   0:00.24 sshd:
ivan at pts/0
25286 root      20   0   51008   1908   1908 S   0.0  0.1   0:00.00 su
 8041 zabbix    20   0   95520   1680   1512 S   0.0  0.1   0:02.10
/usr/sbin/zabbix_agentd: active checks #1 [idle 1 sec]
25240 root      20   0   95368   1620   1572 S   0.0  0.1   0:00.02 sshd:
ivan [priv]
  145 message+  20   0   42892   1164    872 S   0.0  0.1   0:01.55
/usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile
--systemd-activation
 6453 www-data  20   0  125348   1152    656 S   0.0  0.1   0:32.26 nginx:
worker process
20811 postfix   20   0   67640   1136    656 S   0.0  0.1   0:00.86 qmgr -l
-t unix -u
 8038 zabbix    20   0   95520   1084    880 S   0.0  0.1   0:01.04
/usr/sbin/zabbix_agentd: listener #1 [waiting for connection]
 8039 zabbix    20   0   95520    972    768 S   0.0  0.0   0:01.05
/usr/sbin/zabbix_agentd: listener #2 [waiting for connection]
  142 root      20   0   27732    924    636 S   0.0  0.0   0:05.32
/usr/sbin/cron -f
 8040 zabbix    20   0   95520    872    668 S   0.0  0.0   0:01.07
/usr/sbin/zabbix_agentd: listener #3 [waiting for connection]
 8037 zabbix    20   0   93444    728    628 S   0.0  0.0   0:14.57
/usr/sbin/zabbix_agentd: collector [idle 1 sec]
 6462 www-data  20   0  125348    500      0 S   0.0  0.0   0:32.14 nginx:
worker process



As you can see - the cumulative RSS is could barely get to the 100mb.

While this is what `free` returns:

# free -m
              total        used        free      shared  buff/cache
 available
Mem:           2048        1785         261        1750           0
 261
Swap:           512          14         497


So, it clearly states about 85% of ram is occupied.

`slabtop` (due to cgroup limitations?) does not work:

# slabtop
fopen /proc/slabinfo: Permission denied


But if I clear the system caches on the host

echo 3 > /proc/sys/vm/drop_caches


the container memory consumption drops to the expected <100mb.

So the question, how to monitor the memory consumption from the container
reliably? And why does `free` count caches as used memory inside container?



-- 
With best regards, Ivan Kurnosov
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